[science2000]

Right now I have a stainless steel screw in a solutoin of dilute sulfuric acid and sodium chloride, is there any effective way of seperating chromium from iron in solution?
Does chromium, for instance, form a soluble complex with ammonia?

[Alberto_Kravina]

Does chromium, for instance, form a soluble complex with ammonia?

Yes. By adding ammonia you'll precipitate bot Iron and chromium as hydroxides.
Chromium hydroxide is soluble in excess ammonia (Chromium hexammine complex)

[pantone159]

Yes. By adding ammonia you'll precipitate bot Iron and chromium as hydroxides.
Chromium hydroxide is soluble in excess ammonia (Chromium hexammine complex)

Nickel would go with chromium in this case, I think.

[Alberto_Kravina]

Nickel would go with chromium in this case, I think.

Yes! If you want to remove nickel you could do it with dimethylglyoxime (Diacetyldioxime)

[woelen]

By adding ammonia you'll precipitate bot Iron and chromium as hydroxides.
Chromium hydroxide is soluble in excess ammonia (Chromium hexammine complex)

In theory it does, in practice, chromium hydroxide is not soluble in excess ammonia. It takes weeks to dissolve all the chromium hydroxide, this reaction is extremely slow. The solution, obtained, however, is quite nice, it has a nice purple color.

If you want to separate iron from chromium in solution, use sodium hydroxide instead. This forms iron hydroxide and chromium hydroxide. Iron hydroxide does not dissolve when more sodium hydroxide is added, chromium hydroxide, however, dissolves again, forming a deep green solution of chromite ion. Let the precipitate of iron hydroxide settle and decant the green solution.

When nickel or cobalt is present, then these also are separated, these do not form soluble salts in highly alkaline solutions.

The green solution is a very strongly alkaline solution, containing chromium (III). By carefully dripping in acid, until a precipitate is formed again, the chromium (III) can be concentrated. Add so much acid, that a green/grey/blue precipitate is formed and the solution obtains a pH somewhere around 9. Then let the precipitate settle and decant the liquid. The precipitate then can be dissolved in a small amount of acid, to get free aqueous chromium (III) ions.

[science2000]

Will a weak solution of NaOH dissolve chromic hydroxide? I have to custom make solutions of NaOH from Draino, and it's a pain to do.

[woelen]

What do you mean with "weak"? A solution of a few percent by weight of NaOH will indeed dissolve Cr(OH)3 already, but a solution of just 0.1% will not.

[science2000]

Weak as in not too concentrated. I don't think I can continue this experiment, though. I can't seem to dissolve this stainless steel screw at a reasonable rate. I've underestimated how unreactive it is.

[Alberto_Kravina]

How did you try to dissolve the screw?

[woelen]

Try steelwool instead. IIRC it contains quite some chromium as well. When dissolved in hydrochloric acid, it becomes deep green, and I'm quite sure that this color is due to the presence of chromium.

Indeed, metals can be really inert. I've experienced this also. I have some niobium metal and the only way to dissolve this is by adding HF to the solution and even then it only dissolves VERY slowly. Many other metals have this property, also the more common ones like chromium, nickel and iron.

Adding an oxidizer, however, will help dissolve the metal a lot. So use hydrochloric acid, with some hydrogen peroxide added to it and then add the metal. I expect it do dissolve at a higher rate in that case.

[science2000]

I tried dissolving the screw in sodium bisulfate and sodium chloride solution, because that was avaliable to me. Not impressive. I do have muriatic acid, I'll try that.

[pantone159]

I tried dissolving the screw in sodium bisulfate and sodium chloride solution, because that was avaliable to me. Not impressive. I do have muriatic acid, I'll try that.

You should be able to find 3% H2O2 no problem, so do try that as well.

[science2000]

Peroxide will make the chromium form an oxide layer. I have the stainless steel screw in some muriatic acid, it's dissolving away gently. Problem is, I have no idea how much chromium there is. The ferrous ion in solution pretty much has the same color as chromium, a dark green.

[woelen]

No, ferrous ions are not dark green, ferrous ions are almost colorless:

Have a lok at this webpage I made, using very pure chemicals, and be surprised about the color of iron (II) and iron (III) compounds. There are many wrong ideas about the colors of iron compounds in aqueous solution.

http://woelen.scheikunde.net/science/chem/solutions/fe.html